Author's Note: This plot has been done a thousand times before. I gave myself the challenge of putting a Doctor Who quote into a Sherlock fanfic so I wrote this and ended up using two Doctor Who quotes, both from the Twelfth Doctor. Twelve reminds me a lot of Sherlock. I don't own Sherlock or the Doctor Who quotes.
John was feeling introspective one day, not long after he had moved into Baker Street.
"Why did you need a flat mate?" John asked Sherlock as they sat peaceably in their respective armchairs.
Sherlock was on his laptop, presumably typing up another entry for his blog detailing the differences in tobacco ash or something of that nature. "Hm?"
John sighed; due to Sherlock's selective hearing, John repeated himself a lot in their association.
"You clearly don't need a flat mate for monetary reasons, Sherlock," John said, eyeing Sherlock's expensive clothing, multitude of hard-cover, elegant books, and his newest-model laptop. "So, my question was, why did you complain to Mike about needing a flat mate in the first place?"
Sherlock looked up from his computer and frowned at John, but made no answer.
John reflected on how silly it was of him to expect a forthright answer from this great melodramatic enigma of a man. He tried to prompt his friend into an answer.
"I guess there are other reasons for taking a flat mate than monetary reasons. Were you just lonely? Did you need someone to test your experiments on? God knows, I've fulfilled that role." John gave Sherlock a searching glance, wondering if he was getting warmer. Sherlock had gone back to his typing. "Maybe you just needed a carer." John reasoned. "Someone to do your laundry, and buy your groceries, and tell you to eat and sleep before you pass out."
Sherlock chuckled as though John had hit the nail on the head with this last speculation.
John rubbed a hand against his eyes wearily. "Christ, is that the reason you wanted a flat mate? You needed a carer and I fulfilled the job expectations?"
Sherlock huffed another laugh without looking up from his laptop, "Yes, you're my carer. You care so I don't have to."*
The double-meaning of this sentence didn't escape John, and he huffed a laugh, remembering Sherlock's limited capacity to care about what others think, or care about other's wellbeing, and how John often served as Sherlock's moral compass, essentially caring so Sherlock didn't have to.
"So that's my humble role in our alliance? That's the only reason you keep me around?" John asked.
Sherlock closed his laptop and placed it on the little table beside his chair. "Hm. That, and the alternative would be developing a conscience of my own."**
John shook his head and picked up the newspaper lying on the arm of his chair, which he had abandoned at the beginning of this conversation. "Well, that does wonders for my self-esteem." He said, shaking out the creases in the paper.
Sherlock was smiling softly, but John, focused on the paper, didn't see. Sherlock was not an open man. Sherlock had needed a flat mate because Mycroft, overbearingly, chose to withhold Sherlock's share of the Holmes' trust fund until Sherlock was "stable". By stable, the elder Holmes meant clean, possessing no drugs. Mycroft had given Sherlock the money he deserved after John and he had moved into Baker street, and though Sherlock no longer needed a flat mate to pay half of the rent, he didn't inform John. When finding a flat mate, Sherlock hadn't expected to find a best friend. Sherlock knew John was too proud to live on Sherlock's money, so Sherlock let him continue paying half of the rent. Maybe eventually Sherlock would tell John that he was more than just Sherlock's carer.
*Doctor Who Season 8, Episode 2
**Doctor Who Season 8, Episode 6